Elevated carbon dioxide levels may improve growing conditions for plants, but are likely also to amplify frost stress and damage, according to research undertaken for the CRC for Greenhouse Accounting.

Experiments undertaken by Professor Marilyn Ball from the Australian National University indicate that higher levels of carbon dioxide (CO₂) raise the temperature at which frost or ice forms in plants.

At a CRC briefing in Sydney, Professor Ball said frost or ice normally formed on snow gums at minus 5.7 degrees Celsius.

"However, this is raised to minus 4.3 degrees Celsius under higher CO₂ levels," she said.

"In Australia, 80 per cent of the country is affected by frost.

"Even if there are fewer, less severe frosts, there is expected to be similar damage where frosts persist because the higher CO₂ levels cause plants to be more sensitive."

Professor Ball’s experiments in growing snow gums at Bungendore, near Canberra, also indicate that the micro-climate for growing juvenile plants is important in frost-prone areas.

She has found that snow gums seedlings grow best in bare soil, rather than in straw mulch or in grass, in areas were there is frost.

In one study, minimum air temperatures were as much as two degrees Celsius lower above live grass and straw than above bare soil.

"The insulating properties of straw and grass impede the flow of heat from the underlying soil to the overlaying air at night, leading to lower air temperatures above the soil surface.

"Consequently, seedlings which emerge above a grassy layer can be subject to more frequent and more severe frosts."

Professor Ball said a second study with snow gums tested for effects of elevated CO₂.

"Like the first study, growth was best in bare soil.

"Elevated CO₂ did not overcome adverse effects of grass on tree seedling growth, but did enhance growth in bare soil."